The present invention pertains to improvements in the field of electrocoagulation printing. More particularly, the invention relates to an electrocoagulation printing method and apparatus providing enhanced image resolution.
In U.S. Pat. No. 4,895,629 of Jan. 23, 1990, Applicant has described a high-speed electrocoagulation printing method and apparatus in which use is made of a positive electrode in the form of a revolving cylinder having a passivated surface onto which dots of colored, coagulated colloid representative of an image are produced. These dots of colored, coagulated colloid are thereafter contacted with a substrate such as paper to cause transfer of the colored, coagulated colloid onto the substrate and thereby imprint the substrate with the image. As explained in this patent, the positive electrode is coated with a dispersion containing an olefinic substance and a metal oxide prior to electrical energization of the negative electrodes in order to weaken the adherence of the dots of coagulated colloid to the positive electrode and also to prevent an uncontrolled corrosion of the positive electrode. In addition, gas generated as a result of electrolysis upon energizing the negative electrodes is consumed by reaction with the olefinic substance so that there is no gas accumulation between the negative and positive electrodes.
The electrocoagulation printing ink which is injected into the gap defined between the positive and negative electrodes consists essentially of a liquid colloidal dispersion containing an electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent. Where the coloring agent used is a pigment, a dispersing agent is added for uniformly dispersing the pigment into the ink. After coagulation of the colloid, any remaining non-coagulated colloid is removed from the surface of the positive electrode, for example, by scraping the surface with a soft rubber squeegee, so as to fully uncover the colored, coagulated colloid which is thereafter transferred onto the substrate. The surface of the positive electrode is thereafter cleaned by means of a plurality of rotating brushes and a cleaning liquid to remove any residual coagulated colloid adhered to the surface of the positive electrode.
When a polychromic image is desired, the negative and positive electrodes, the positive electrode coating device, ink injector, rubber squeegee and positive electrode cleaning device are arranged to define a printing unit and several printing units each using a coloring agent of different color are disposed in tandem relation to produce several differently colored images of coagulated colloid which are transferred at respective transfer stations onto the substrate in superimposed relation to provide the desired polychromic image. Alternatively, the printing units can be arranged around a single roller adapted to bring the substrate into contact with the dots of colored, coagulated colloid produced by each printing unit, and the substrate which is in the form of a continuous web is partially wrapped around the roller and passed through the respective transfer stations for being imprinted with the differently colored images in superimposed relation.
The positive electrode which is used for electrocoagulation printing must be made of an electrolytically inert metal capable of releasing trivalent ions so that upon electrical energization of the negative electrodes, dissolution of the passive oxide film on such an electrode generates trivalent ions which then initiate coagulation of the colloid. Examples of suitable electrolytically inert metals include stainless steels, aluminium and tin.
As explained in Applicant's U.S. Pat. No. 5,750,593 of Mar. 12, 1998, the teaching of which is incorporated herein by reference, a breakdown of passive oxide films occurs in the presence of electrolyte anions, such as Cl.sup.-, Br.sup.- and I.sup.-, there being a gradual oxygen displacement from the passive film by the halide anions and a displacement of adsorbed oxygen from the metal surface by the halide anions. The velocity of passive film breakdown, once started, increases explosively in the presence of an applied electric field. There is thus formation of a soluble metal halide at the metal surface. In other words, a local dissolution of the passive oxide film occurs at the breakdown sites, which releases metal ions into the electrolyte solution. Where a positive electrode made of stainless steel or aluminium is utilized in Applicant's electrocoagulation printing method, dissolution of the passive oxide film on such an electrode generates Fe.sup.3+ or Al.sup.3+ ions. These trivalent ions then initiate coagulation of the colloid.
As also explained in Applicant's U.S. Pat. No. 4,895,629, the negative electrodes must be spaced from one another by a distance which is equal to or greater than the electrode gap in order to prevent the negative electrodes from undergoing edge corrosion. This considerably limits the resolution of the image printed by electrocoagulation so that an image resolution of more than about 200 lines per inch cannot be obtained.
Applicant has attempted to increase the image resolution while satisfying the above minimum distance between the negative electrodes by arranging the electrodes along two closely adjacent parallel rows with the negative electrodes of one row being staggered with respect to the negative electrodes of the other row. Upon electrical energization of these electrodes, Applicant has observed that there is a grouping between the dots of coagulated colloid formed opposite the electrode active surfaces of the energized electrodes of one row and those formed opposite the electrode active surfaces of the energized electrodes of the other row, resulting in dots having an elliptical configuration rather than the desired circular configuration.